1. Field of the Invention
The present invention is directed to a blur correcting apparatus and method used for cameras which prevent an image on a film plane from blurring due to a displacement of an optical axis of a photographing optical system of a camera, caused, for example, by camera shakes.
2. Description of Related Art
When a picture is taken by a camera, in particular, when the camera is being hand held without using a tripod, an accidental movement of the optical axis of the photographing optical system of the Camera tends to occur due to camera shakes which are caused particularly when the camera is not held stably by the photographer, a picture of a dark object is taken at a slow shutter speed or the photographer takes a picture while moving, etc. The blurred picture is not clearly seen due to the streaked image. The blur caused by the camera shake can be eliminated to some extent, by the hardware solution of using a bright lens, or increasing the film sensitivity to increase the shutter speed, or the software solution of improving the photographer's skill.
A blur correcting apparatus which can prevent the occurrence of blurring due to camera shakes has been proposed, in which the angular velocity or the acceleration of the camera is detected and the blur correcting lens is moved in an opposite direction to the direction of the camera shake to prevent a movement of the image on the film plane.
When this type of blur correcting apparatus is used, in which the occurrence of blurring can be prevented by detecting the camera shake by means of an angular velocity or acceleration sensor, the value of the angular velocity or acceleration detected by the sensor does not turn out to be zero due to the difference of the actual voltage from the theoretical demand voltage. This will occur, for example, when there is a DC (direct current) voltage present when there is no camera shake, i.e., when the sensor is supposed to detect no angular velocity or acceleration.
The above noted DC voltage error is defined as the null voltage when the acceleration and velocity is zero but the sensor signal indicates a voltage. For this reason, when an angular velocity or acceleration signal is converted to a deviation amount of the blur correcting lens, the angular velocity or acceleration signal is converted together with the DC voltage error. Since an angular velocity or acceleration signal is usually quite small, a larger error than the null voltage error occurs in the signal for blur correction when the angular velocity or acceleration signal is amplified together with such a DC output as DC voltage error. In the blur correcting apparatus, the blur correcting lens is moved in the opposite direction to that of the camera shake to prevent a movement of the image on the film plane, in accordance with an angular velocity or acceleration measured by the blur correcting apparatus. Because of this structure, in the blur correcting apparatus, when the above-noted large error occurs, the occurrence of blurring can not accurately be prevented.
In order to overcome the foregoing problem, generally, such a circuit as a DC blocking filter circuit (CR direct current blocking filter circuit) consisting of a condenser C.sub.o and a resistor R.sub.o (shown in FIG. 8) is installed in a blur correcting apparatus. This DC filter circuit is a high-pass filter, and therefore, it allows a high frequency component to pass while removing a low frequency component including such a direct current component as the DC voltage error. However, the DC blocking filter circuit also removes some low frequency components of the output of the angular velocity or acceleration sensor, which is generated due to an actual camera shake and thus should not to be removed at the same time. When the time constant (C.sub.o R.sub.o) of the DC blocking filter is especially small, the range of the frequency band through which a low frequency component is allowed to pass is moved to a higher frequency, thereby the above low frequency component of an output generated due to an actual camera shake is incorrectly removed. For this reason, according to the blur correcting apparatus having such a circuit as the above noted DC blocking filter circuit means that an accurate output of the angular velocity or the acceleration sensor cannot be obtained even though such a direct current component as the DC voltage error is removed. Consequently, the output signal for controlling the blur correcting lens would still be imprecise.
Provided the time constant of the DC blocking filter shown in FIG. 8 is "T" and the cutoff frequency of the same is "fc", since the time constant "T" and the cuttoff frequency "fc" are represented by "C.sub.o .times.R.sub.o " and "1/(2.pi..times.T)", respectively, the cutoff frequency "fc" is represented by the following equation: EQU fc=1/(2.pi..times.C.sub.o .times.R.sub.o)
and the transmission function of the DC blocking filter circuit is represented by the following equation: EQU VO/VI=T.sub.s /(T.sub.s +1)
wherein "VI" represents an input voltage and "VO" represents an output voltage.
If the frequency of a camera shake which is subject of blur correction falls within a low frequency band, for instance, a band ranging from approximately 0.5 Hz (low limit) to 20 Hz (top limit), since fc=1/(2.pi..times.T), the time constant T is 0.3 seconds. In the case where the shutter speed is a slow speed for instance, about 1 to 2 seconds, it is necessary to make the above low limit even lower. For this reason, it takes a long time approximately ten times longer than the time constant T, for the direct current output component to be approximately zero and therefore having substantially no influence upon the result of the angular velocity or acceleration integral. Therefore, immediately after the main switch of the camera is turned ON or the camera is panned in one direction by a great amount for the purpose of determining or changing the composition of a picture, it takes a long time for the residual direct current output to fall to substantially zero. Therefore, it is possible for a photographer to miss a good photograph during the time he or she is waiting for the direct current output component to fall to substantially zero.